As is well known, exhaust temperature is lower in internal combustion engines such as diesel engines in which combustion occurs in an excessive oxygen (lean) state than in gasoline engines in which combustion occurs in a stoichiometric state. The low exhaust temperature makes activation of an exhaust purification catalyst difficult; the exhaust purification catalyst is, for example, an NOx catalyst and is installed in an exhaust passage. In particular, in an internal combustion engine mounted in a vehicle, an exhaust purification catalyst is installed away from a combustion chamber of the internal combustion engine or installed under the floor of the vehicle and thus exposed to a wind flow. Thus, disadvantageously, increasing the temperature of the exhaust purification catalyst is difficult, resulting in the difficulty of activating the exhaust purification catalyst. Furthermore, there has been a demand to activate the exhaust purification catalyst earlier during the cold start of the internal combustion engine.
To prevent this, such a heated gas generation apparatus as described below has been provided. The apparatus includes a first catalyst and a second catalyst each provided in an exhaust passage located upstream of the exhaust purification catalyst, the first and second catalysts each providing an oxidation function, and a fuel supply valve allowing fuel to be supplied to the first catalyst. The supplied fuel is sequentially combusted by the first and second catalysts to generate hot heated gas. The generated heated gas can be used to heat the downstream exhaust purification catalyst.
On the other hand, as an exhaust purification catalyst, a selective reductive NOx catalyst (what is called urine SCR) is known which continuously reduces NOx in exhaust gas using a urea aqueous solution serving as a reducing agent. In an exhaust purification apparatus using the selective reduction NOx catalyst, the urea aqueous solution is evaporated by exhaust heat or catalytic heat and thus hydrolyzed to generate ammonia. The ammonia and NOx react with each other in the catalyst to reduce and purify the NOx.
The selective reduction NOx catalyst is expected to be heated by heated gas generated by the heated gas generation apparatus. However, in this case, when hydrocarbon HC that is a combustion residue of fuel is discharged from the heated gas generation apparatus, the discharged HC may disadvantageously suppress the reaction between ammonia and NOx in the NOx catalyst or poison the NOx catalyst with HC. Thus, the amount of fuel supplied needs to be controlled so as to prevent the heated gas generation apparatus from discharging HC.
Patent Document 1 discloses that in order to prevent the selective reduction NOx catalyst from being poisoned with SOF, the amount of reducing agent supplied is determined so as to set the concentration of HC to the upper limit value at which the NOx catalyst is prevented from being poisoned with HC.
However, performing only such fuel supply amount control is insufficient. That is, if the catalyst in the heated gas generation apparatus is poisoned with HC or degraded and thus fails to fulfill its inherent performance, even when the fuel supply amount is accurately controlled, part of the fuel remains unburned, with HC discharged.
Thus, the present invention has been developed in view of the above-described circumstances. An object of the present invention is to provide an exhaust purification apparatus for an internal combustion engine which can detect poisoning of the catalyst in the heated gas generation apparatus with HC or degradation of the catalyst to prevent the heated gas generation apparatus from discharging HC.